This invention relates to methods and apparatus for packing chromatography columns with the particulate medium used for chromatography.
It is no simple matter to pack particulate chromatography media into chromatography columns, particularly the large columns (e.g. from 100 mm up to as much as 2000 mm and beyond in diameter) used in industrial-scale preparative chromatography. Typical requirements are that the packed bed be continuous, uniform, fill the column space entirely and be sufficiently initially compressed not to shift under the fluid flow conditions prevailing subsequently during chromatography. An improperly packed bed is practically useless and has serious cost implications having in mind the usually high value of products purified using chromatography.
The conventional method of packing a column is to remove the top wall (end plate or piston) of the column and pour in the medium as a slurry in a suitable liquid vehicle. The vehicle flows down through the separation filter (sinter) at the foot of the column and away through a waste port, while the bed of solid medium (e.g. silica) gradually accumulates to fill the column space. This is a skilled and time-consuming task.
For this reason the use of automated packing by way of a packing port through the column wall has recently been preferred. The packing port, fed with a slurry of the medium via a feed conduit, may be a hole with a simple open/shut control. It is known for example to open and shut the hole vis à vis the column interior using a piston-fashion movement of the column end cell to cover and uncover it. The packing port may have a nozzle or a more sophisticated nozzle or spray nozzle arrangement. See for example GB-A-2258415 and WO-A-96/10451. These describe arrangements in which slurry enters the column space at a spray nozzle having an array of spray holes to distribute the incoming flow. Another feature is that the nozzle projects from the column wall into the column space, at least at the time of spraying the slurry. The present proposals are preferably used with spray nozzle arrangements having one or both of these features.
The introduction of slurry through a restricted packing port raises a number of important practical requirements, largely associated with the need to feed the slurry at substantial pressure in order to achieve a flow rate adequate to distribute the slurry and pack the bed down. Depending on the circumstances various difficulties can arise, including inadequate levels of packing, damage to the media particles (especially with large particle sizes), and separation of the slurried particles from the liquid slurrying vehicle in the pressurised feed system.
The present proposals aim to provide new and useful methods and apparatus for packing particulate chromatography media into chromatography columns through a packing port in the column wall, preferably through a nozzle or spray nozzle.
In general terms what we propose is to establish a pressurised motive flow of liquid vehicle along a feed conduit leading to the packing port, typically by means of a pump, and to introduce a separate flow of the particulate medium into the feed conduit to be entrained in the motive flow. The medium is preferably introduced into the feed conduit as a dispersion in a liquid vehicle, which may be the same as that used for the motive flow.
The motive flow of liquid vehicle can then be substantially or entirely free of particulate medium upstream of the flow junction at which it entrains the medium.
Because the motive flow can provide the pressure necessary for packing, the introduced flow of particulate medium can be at a lesser pressure head. This avoids the need to subject the medium itself to pumping at packing pressure, with possible mechanical degradation of the particles, or to any pumping at all, and may enable better packing results particularly with large-particle media which are most liable to be damaged in a pump.
A further specific advantage can be achieved in cases where the medium and liquid vehicle are poorly compatible. This may arise in particular where one or more of the following apply:
the medium and liquid vehicle are widely different in polarity;
the medium particles are large;
the liquid vehicle is of low viscosity.
An example is a slurry of normal-phase silica in organic solvent, especially non-polar organic solvent. We find that our techniques can enable effective packing of medium/vehicle combinations which, if pumped as slurry, will separate. This is valuable because of the general demand for a wide choice of solvents as eluents in chromatography; it is highly preferred to slurry the packing using the same solvent.
However the method is useful for a wide range of media. For example it can avoid subjecting soft or brittle media to the action of a mechanical pump. Possible media include rigid, semi-rigid and softer media such as all silicas, polystyrenedivinylbenzene, polymethacrylates, agarose, dextran and cellulosic media.
We prefer to introduce the flow of particulate medium into the established pressurised motive flow at a reduced-pressure zone of the feed conduit, and most preferably at by means of an ejector (jet pump) provided therein. The ejector is a well-known fluid pumping arrangement in which a flow of a motive fluid at a higher pressure is used to pump another fluid at a lower pressurexe2x80x94conventionally called the suction flowxe2x80x94at some intermediate resultant discharge pressure. An ejector typically has a relatively restricted nozzle at which a fast flow of the motive fluid emerges into a larger cross-sectional area region where it can accelerate the suction flow; normally this is followed by a gradual increase of cross-sectional area (a diffuser) to restore the pressure of the combined flows by reducing the kinetic energy. The fluid mechanics and practical implementation of ejector operation are well established and need not be discussed in detail.
By thus exploiting a low-pressure zone in the motive feed conduit, a supply of particulate medium can be entrained from a medium source having a passive or non-pumped feed, e.g. under gravity. It may be drawn up against a negative pressure head.
It is preferred that liquid vehicle emerging from the column during packing is recycled to the packing process. The process is apt to introduce the medium at a high slurry dilution and recycling helps to reduce the overall volume of liquid vehicle required.
A continuous (steady) output pump such as a centrifugal pump is preferred as the motive device because it can provide a steady drawing pressure for the packing medium; pulsing pumps such as diaphragm pumps will work but are less preferred.
Aspects of the invention include methods as described and also apparatus adapted to carry out such methods. Such apparatus may comprise for example a feed conduit connected at an upstream end to a pump or other fluid flow motive means, and at its downstream end having a connector for connection to a slurry spray nozzle or other slurry port provided through the wall of a chromatography column. A medium conduit meets the feed conduit at an intermediate junction thereof, for introduction of particulate mediumxe2x80x94preferably as a slurry of the medium in liquid vehiclexe2x80x94to be entrained in the pressurised flow of liquid vehicle passing along the feed conduit from the motive means. The junction preferably comprises means such as an ejector for providing a low-pressure zone as described above, helping to draw the flow of medium into the pressurised flow.
The apparatus may further include one or both of a liquid vehicle supply vessel connected to the motive means and a medium supply vessel connected to the medium supply conduit, preferably directly without any mechanical motive means. Desirably means are provided for feeding liquid vehicle back to the medium supply vessel. Means for agitating the vessel contents to maintain the slurry are usually provided. This may be by hydraulic mixing, e.g. feeding a recycled liquid flow up into the bottom of the container to create turbulence, and/or a mechanical agitator for stirring or agitating the contents of that vessel to maintain slurry therein may be used. The apparatus may include a recycle line for returning liquid vehicle to one or both of the liquid vehicle supply vessel and the medium supply vessel after it has passed through the column during packing. This can reduce the volume of liquid used. It can also enable a smaller supply vessel to be used because slurrying-up is ongoing and the medium can be added in convenient increments. One or more chromatography columns, each having the spray nozzle or other packing orifice and an outlet for liquid vehicle which has flowed through the column from the packing orifice, may also be included in the apparatus. Elements of the apparatus may be mounted on a common frame which preferably is mobile i.e, as a trolley or xe2x80x98skidxe2x80x99 so as to be movable to and from a column needing to be packed.